The present invention relates to an electrochemical gas sensor and a method for determining gaseous components.
The European Patent Application No. 0 678 740 discusses a gas sensor for determining the NOx concentration in a gas mixture, in which two measuring-gas compartments, each having a pump cell, are arranged one behind the other in one layer plane of a planar, oxygen-ion-conducting, ceramic carrier. The measuring gas streams via a first diffusion opening into the first measuring-gas compartment in which a first inner pump electrode is arranged. An outer pump electrode is exposed directly to the exhaust-gas compartment. The first inner pump electrode and the outer pump electrode form the first pump cell. A predetermined oxygen partial pressure is adjusted in the first measuring-gas compartment by pumping oxygen in and out with the aid of the first pump cell. A concentration cell (Nernst cell) has a measuring electrode and a reference electrode in communication with an air atmosphere, the measuring electrode being disposed in the first measuring-gas compartment. To set a constant oxygen partial pressure in the first measuring-gas compartment, the electric voltage (electromotive force) of the concentration cell is adjusted to a constant value by a pump voltage of the first pump cell. The first and second measuring-gas compartments are connected with a connecting duct which represents a further diffusion opening, the atmosphere that is set to a constant oxygen partial pressure being diffused into the second measuring compartment via the connecting duct. Disposed in the second measuring-gas compartment is a further inner pump electrode which cooperates with the reference electrode arranged in the air-reference duct and forms the second pump cell. The further inner pump electrode is constructed of a material, e.g., of rhodium, which produces a decomposition of NO to N2 and O2. The reduced oxygen obtained at the further inner pump electrode is pumped in ion form via an applied pump voltage to the reference electrode and is released there into the air atmosphere. Since the atmosphere in the first measuring-gas compartment is retained at a constant oxygen partial pressure, the pump current for evacuating the reduced oxygen from the second measuring-gas compartment is proportional to the NOx concentration.
The sensor-element design is believed to be relatively complicated and the measuring accuracy is believed to be dependent on many factors, such as the exact adjustment of the measuring temperature and the oxygen partial pressure.
The exemplary gas sensor and the exemplary method of the present inventions are believed to have the advantage that, because of the sensor design and the measuring method utilized, the concentration of the gaseous component to be measured is yielded in a very simple manner which is nevertheless sufficiently accurate. The exemplary gas sensor according to the present invention includes a first pump cell and two additional measuring pump cells likewise functioning according to the amperometric measuring principle. Since both measuring pump cells are operated according to the same amperometric measuring principle, a particular advantage of the exemplary device and exemplary method of the present inventions is believed to involve the fact that a measuring signal proportional to the concentration of the gaseous components to be determined is obtained with great accuracy through simple subtraction of the pump currents of both amperometric measuring pump cells.
In other exemplary embodiments, the spatially confined arrangement of the two electrochemical measuring pump cells allows a simple and temperature-compensated measurement. Furthermore, the common outer pump electrode of the two measuring pump cells can be arranged in direct contact with the ambient air, but for that purpose, the integration of an air-reference duct into the layer system of the sensor element may be necessary. However, the measuring based on determining pump currents also permits the arrangement of the outer pump electrode in the same atmosphere as the measuring-gas, and consequently allows for the omission of an air-reference duct. It is believed that this should simplify the design of the sensor element substantially, and should represent a great cost savings.